Key message
Inequalities exist in cancer survival when examined by income quintile. Lower-income populations were shown to have poorer survival for breast, colorectal, lung and prostate cancers.
Background
Monitoring and reporting on cancer survival provides a mechanism for understanding the effectiveness of Canada’s cancer care system. Many factors can influence the likelihood of surviving cancer, including adequate access to effective screening, timely diagnosis and effective treatment. There is substantial evidence that cancer survival varies by socioeconomic status (SES), possibly because of disparities in access to high-quality care in low-income populations.1
This Special Feature provides an overview of net survival by SES (measured by the average income of the patient’s neighbourhood relative to the overall population) for several cancers in adults aged 15–99 at diagnosis, including breast, lung, colon and rectum (combined), prostate and stomach, as well as acute lymphoblastic leukemia in children (aged up to 14 years). The aim is to identify survival disparities among different income groups so that cancer control strategies can be targeted to reach populations at risk of poorer outcomes.
What we know about disparities in cancer control in Canada: A look at previous work.
The influence of income (among other socio-demographic factors) on access to cancer control services was examined in both the 2014 Examining Disparities in Cancer Control: A System Performance Special Focus Report and the 2015 Cancer System Performance Report, produced by the Canadian Partnership Against Cancer (the Partnership). Key findings include the following:
- Screening. Self-reported screening participation rates for breast, cervical and colorectal cancers were highest in high-income populations.2
- Stage-specific incidence. Women with higher incomes were more likely than lower-income women to have their cancer diagnosed at an early or intermediate stage.3
- Wait times. Low-income populations generally had longer wait times for resolution of an abnormal breast screening result.3
- Treatment. There was no definitive difference by income in access to or use of radiation therapy (as measured by radiation therapy wait times and radiation therapy utilization), but there were differences in breast cancer treatment patterns by income (as measured by mastectomy rates).3
Information on net survival by SES adds to the evidence on disparities in cancer control in Canada. It allows us to examine the extent to which variations in cancer control activities affect survival in different population groups (defined in this case by income). |
Methods
Five-year net survival was estimated for Canadian adults (aged 15–99) diagnosed with one of nine cancers (breast, lung, colon and rectum combined, prostate, liver, ovary, cervix, stomach and leukemia) and for children (aged up to 14 years) diagnosed with acute lymphoblastic leukemia between 2004 and 2009. Estimation was done as a sub-analysis of the CONCORD-2 study, conducted specifically for Canada and funded by the Partnership. The CONCORD-2 study is the most comprehensive study to date on international comparisons of population-based cancer survival.4
In this study, SES was defined by average neighbourhood income and was derived from PCCF+ version 5K, based on the 2006 census, using patients’ full postal codes. Income quintiles were obtained by ranking the average household-size adjusted measure of household income per dissemination area. These quintiles were community-specific.
Net survival was estimated at one, three and five years after diagnosis, by income quintile and overall. The International Cancer Survival Standard age groups and weights were applied for the adult malignancies. For childhood leukemia, equal weights were applied to each of the three age groups (0–4, 5–9 and 10–14 years). Provincial life tables specific for each sex, income quintile and calendar year were used to control for background mortality. Net survival for cancer patients was controlled for the widely different levels of background mortality by age and sex in the general population within each income quintile in each province. Therefore, variations by income in mortality not related to cancer (such as cardiovascular disease) were already factored into the baseline survival and would, therefore, not skew the net cancer survival rates.
Data were contributed by 10 provincial cancer registries, each of which covers the entire population of the province. Income quintile data were not available for Newfoundland and Labrador at the time of this analysis, meaning that survival could not be estimated by SES for this province. Populations were too small in the Northwest Territories, Nunavut and Yukon to enable accurate estimation of life tables and net survival by SES.
In this Special Feature, five-year net survival by income quintile is presented for the four most common cancers in Canada: breast, lung, colorectal and prostate cancers, as well as for stomach cancer and childhood leukemia.
Results
Five-year net survival for adults was highest in high-income populations (Q5) for breast, colorectal, lung and prostate cancers (Figure 8.24). The same survival gradient by income existed for cancers of the liver and ovary, as well as for leukemia in adults (data not shown).
By contrast, five-year net survival for stomach cancer and childhood leukemia did not exhibit a strong gradient by income—survival was similar across quintiles (Figure 8.24).
Conclusions
Lower-income populations were shown to have poorer survival for most of the cancers reported here. There is evidence in the literature that lower-income patients are less likely to have their symptoms recognized and investigated early, resulting in a more advanced stage at diagnosis, when treatment is less effective and, ultimately, in a poorer prognosis.5-7 Additionally, poorer access to screening or early detection and treatment, both in terms of timeliness and quality of care (i.e., lower-income populations may be receiving poorer-quality, less timely care) may affect survival outcomes.1, 8 For instance, low-income cancer patients often have longer wait times between an abnormal screening result or the detection of symptoms and receipt of follow-up care or treatment, both in Canada and the United States3, 5. The extent to which these differences in survival are influenced by differences in screening and early detection (or early presentation) and/or treatment effectiveness could be explored in the future by examining stage-specific survival separately in each income quintile.
Survival for stomach cancer and childhood leukemia does not conform to this pattern: there appears to be no clear relationship between income and survival. Survival for both stomach cancer and childhood leukemia has been shown to be associated with SES in other research9-13. The fact that this association does not appear to be the case in Canada should be celebrated, particularly in the case of childhood leukemia, and potentially merits further exploration into possible factors that may be yielding more equitable access to care and relatively comparable survival among income quintiles for these two cancers that could be applied to others.
Identifying both the existence of disparities and the magnitude of the survival gap for different cancers is the first step toward addressing the inequality in cancer survival seen across the country. Targeted cancer control strategies could then be developed to promote knowledge and to improve access to timely and effective care for patients across the socio-demographic spectrum, leading to more equitable outcomes.
Upcoming publication of data from CONCORD-2 on survival by socioeconomic status
Detailed results for each of the 10 cancers studied were provided by the CONCORD Central Analytic Team to the Partnership; further exploration into survival disparities by income in Canada are underway. Future publications will include examination of the survival gradient by income provincially and by age group for all 10 cancers, as well as identification of the influence of factors such as stage at diagnosis. |
References
- Halpern MT. Cancer disparities research: it is time to come of age. Cancer. 2015 Apr 15;121(8):1158-9.
- Canadian Partnership Against Cancer. The 2015 Cancer System Performance Report. Toronto (ON): Canadian Partnership Against Cancer; 2015 Jun. 161 p.
- Canadian Partnership Against Cancer. Examining disparities in cancer control: A system performance special focus report. Toronto (ON): Canadian Partnership Against Cancer; 2014 Feb. 83 p.
- Allemani C, Weir HK, Carreira H, Harewood R, Spika D, Wang XS, et al. Global surveillance of cancer survival 1995-2009: analysis of individual data for 25 676 887 patients from 279 population-based registries in 67 countries (CONCORD-2). Lancet. 2014 Nov 26;385(9972):977-1010.
- Centers for Disease Control and Prevention. Surveillance of screening-detected cancers (colon and rectum, breast and cervix) – United States, 2004-2006. Atlanta (GA): Centers for Disease Control and Prevention; 2010 Nov. 26 p.
- Wang F, McLafferty S, Escamilla V, Luo L. Late-Stage Breast Cancer Diagnosis and Health Care Access in Illinois. Prof Geogr. 2008 Feb;60(1):54-69.
- Late-stage cancer detection in the USA is costing lives. Lancet. 2010;376(9756):1873.
- Byers TE, Wolf HJ, Bauer KR, Bolick-Aldrich S, Chen VW, Finch JL, et al. The impact of socioeconomic status on survival after cancer in the United States : findings from the National Program of Cancer Registries Patterns of Care Study. Cancer. 2008 Aug 1;113(3):582-91.
- Siemerink EJ, Hospers GA, Mulder NH, Siesling S, van der Aa MA. Disparities in survival of stomach cancer among different socioeconomic groups in North-East Netherlands. Cancer Epidemiol. 2011 Oct;35(5):413-6.
- Wu CC, Hsu TW, Chang CM, Yu CH, Wang YF, Lee CC. The effect of individual and neighborhood socioeconomic status on gastric cancer survival. PLoS One. 2014;9(2):e89655.
- Njoku K, Basta N, Mann KD, McNally RJ, Pearce MS. Socioeconomic variation in survival from childhood leukaemia in northern England, 1968-2010. Br J Cancer. 2013 Jun 11;108(11):2339-45.
- Petridou ET, Sergentanis TN, Perlepe C, Papathoma P, Tsilimidos G, Kontogeorgi E, et al. Socioeconomic disparities in survival from childhood leukemia in the United States and globally: a meta-analysis. Ann Oncol. 2015 Mar;26(3):589-97.
- Gupta S, Wilejto M, Pole JD, Guttmann A, Sung L. Low socioeconomic status is associated with worse survival in children with cancer: a systematic review. PLoS One. 2014;9(2):e89482.
